This shit flourishes because people want it. There will be thousands more sites like that, shared between like-minded individuals.
If you link to things that creates potential liability (yes you might be able to put forward a defence, but that's not really a stage that you want to get to).
And secondly, if you view illegal images they will be on your computer in the browser cache - you are in possession of them as is made clear on that page, and they will remain on your computer for a considerable period of time due to modern hard drives.
Again, you may be able to put forward a defence to that but again that's not really a stage you would want to get to.
Finally starting threads on mumsnet, twitter, etc. does not constitute reporting it.